hsyuan opened a new pull request #2051:
URL: https://github.com/apache/calcite/pull/2051

   If the child subset is used to derive new traits for current relnode, the
   subset will be marked REQUIRED when registering the new derived relnode and
   later will add enforcers between other delivered subsets.  e.g. a MergeJoin
   request both inputs hash distributed by [a,b] sorted by [a,b]. If the left
   input R1 happens to be distributed by [a], the MergeJoin can derive new 
   from this input and request both input to be distributed by [a] sorted by
   [a,b]. In case there is a alternative R2 with ANY distribution in the left
   input's RelSet, we end up with requesting hash distribution [a] on 
   R2, which is unnecessary and waste, because we request distribution by [a]
   because of R1 can deliver the exact same distribution and we don't need to
   enforce properties on other subsets that can't satisfy the specific trait

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